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Giving evidence from behind a screen on the second day of Jasulaitiene's trial, an undercover officer, who can only be known as R21, told the court he was hiding behind a wall as part of a training exercise when the crash took place.

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He said the motorbike was "in pieces" and PC Dixon was "five to 10 feet away."

He said other members of the operation arrived in their vehicles within minutes of the crash.

The counter surveillance exercise

R21 told the jury he and PC Dixon were just two of a number of police and HMRC staff taking part in a "counter surveillance" training exercise in the area when the crash took place at around 1.50pm on Tuesday, December 5, 2017.

The idea of the exercise was to train the HMRC staff for working undercover abroad. The aim was for one team to try to survey the other, and the other team to avoid being surveyed.

He said PC Dixon was riding a "surveillance vehicle", which he said he believed had been specifically designed not to be seen, and PC Dixon was not wearing high-visibility clothing at the time of the crash.

PC Dixon was 'accelerating'

PC James Dixon, 39, died while on duty (Image: Thames Valley Police)

R21 told the jury he had seen PC Dixon "accelerating" as he went past where he was stationed as part of the operation.

During questioning from defence lawyer Ian Bridge, the officer said it was often the case police vehicles involved in training exercises travel over the speed limit as the law states they are allowed to do.